In recent years, research and development of light-emitting elements utilizing electroluminescence (EL) have been actively carried out. As a basic structure of the light-emitting element, a layer containing a light-emitting substance is sandwiched between a pair of electrodes. Voltage is applied to the light-emitting element to obtain light emission from the light-emitting substance. Use of a substrate having flexibility (also referred to as a flexible substrate) for a light-emitting device with the light-emitting element has been considered to make the light-emitting device flexible. As a method for manufacturing a light-emitting device using a flexible substrate, a technology in which a separation layer is formed over a substrate such as a glass substrate or a quartz substrate, a semiconductor element such as a thin film transistor is formed over the separation layer, and then, the semiconductor element is transferred to another substrate (e.g., a flexible substrate) has been developed (see Patent Document 1).
When components of a light-emitting device or the like are formed directly on a flexible substrate, the upper-limit temperature of a manufacturing process needs to be set relatively low because a material for the flexible substrate has low heat resistance. For this reason, the quality of the components of the light-emitting device might be reduced. Furthermore, in the case where alignment is required in the manufacturing process, expansion and contraction of the flexible substrate due to heating in the manufacturing process might reduce the yield. Accordingly, to reasonably perform various heating steps, alignment steps, and the like in a manufacturing process of a light-emitting device or the like using a flexible substrate, it is preferable that the steps be performed on a rigid substrate such as a glass substrate, and that the components of the light-emitting device or the like be transferred to the flexible substrate in the final stage of the manufacturing process.
In addition, depending on the kinds of light-emitting device or the like using a flexible substrate, the following manufacturing process can be used: thin components (including, for example, a light-emitting element) formed over two different rigid substrates are attached to each other, one of the rigid substrates is separated to be replaced by a flexible substrate, and the other of the rigid substrates is also separated to be replaced by a flexible substrate. In this process, the first separation step requires a technique with great difficulty for separating one rigid substrate from the pair of rigid substrates attached to each other with an extremely narrow gap. In order for this separation to be performed, a method in which a cut is formed in the separation layer with a blade, separation is induced by blow of a gas from the cut, and a rigid substrate is drawn up by suction pads such that the separation extends to the whole area of the separation layer has also been proposed (see Patent Document 2). However, in the method of pulling the substrate apart by the suction pads, force required for pulling apart is not necessarily uniform and varies as the separation proceeds. Nonetheless, with the method disclosed in Patent Document 2, it is impossible to apply delicate, adjusted force to a glass substrate to be separated.